1. Field of the Invention
The present invention relates generally to the field of semiconductor wafer processing and, more particularly, to the polishing of semiconductor wafers utilizing chemical mechanical polishing technologies using polishing pads.
2. Background of the Related Art
The advances in integrated circuit device technology have necessitated the advancement of chemical mechanical polishing (CMP) technology to provide better and more consistent surface planarization processes. The manufacture of these devices (i.e. complementary metal oxide semiconductors (CMOS), very large scale integration (VLSI), ultra-large scale integration (ULSI), microprocessors, semiconductor memory, and related technologies) on prepared substrates and the preparation of the substrates themselves (prime wafer polishing) require very highly planar and uniform surfaces. To achieve these high levels of planarity and uniformity of surface preparation the processes that produce them must be reliably and consistently performed. Surfaces that are underpolished, overpolished, nonuniform, and/or nonplanar will not produce quality microelectronic devices.
Chemical mechanical polishing processes are used extensively at various stages of integrated circuit processing to planarize surfaces of semiconductor wafers, such as silicon or gallium-arsenide, including prime polishing to prepare the wafers before they enter the device fabrication process. This technology is also employed in polishing optical surfaces, geological samples, metal substrates, as well as other semiconductor-based substrates.
During the device fabrication processes, the wafers may undergo multiple masking, etching, implantation, and dielectric and conductor deposition processes. Many of these processes, especially deposition and etching, may produce a surface that is highly irregular and nonplanar. Due to the high-precision required in the photolithographic steps during the production of these integrated circuits, an extremely flat surface is generally needed on at least one side of the semiconductor wafer to ensure proper accuracy and performance of the microelectronic structures created at the wafer surface. Therefore, between each processing step, it is usually necessary to polish or planarize the surface of the wafer to obtain a substrate surface having a high degree of planarity and uniformity of material removal.
In CMP fabrication techniques, a free abrasive chemical slurry is often used along with a rotating polishing pad, linear polishing belt, or rotating drum to contact the workpiece surface and to polish and planarize that surface. Typical examples of these types of apparatus are described in U.S. Pat. No. 5,329,732, assigned to SpeedFam disclosing a rotating polishing pad polisher; PCT Publication WO 97/20660, assigned to Applied Materials disclosing a linear belt polisher; and U.S. Pat. No. 5,643,056, assigned to Ebara Corporation and Kabushiki Kaisha Toshiba disclosing a rotating drum polisher. The disclosures of the foregoing patents, in relevant part, are incorporated herein, by reference.
In exemplary prior art polishing methods, one side of the wafer is attached to a wafer carrier and the other side of the wafer is pressed against a polishing surface. In general, the polishing surface comprises a polishing pad or belt that can be formed of various commercially available materials such as blown polyurethane. Typically, a water-based colloidal abrasive slurry such as cerium oxide, aluminum oxide, fumed/precipitated silica or other particulate abrasives is deposited upon the polishing surface. During the polishing or planarization process, the workpiece (e.g., silicon wafer) is typically pressed against the moving (e.g., rotating or linearly translating) polishing surface. In addition, to improve the polishing effectiveness, the wafer may also be rotated about its vertical axis and/or oscillated over the inner and outer radial surface of the polishing surface.
When pressure is applied between the polishing pad and the workpiece being polished, mechanical stresses and the abrasive particles within the slurry create mechanical strain on the chemical bonds that form the surface being polished. These stresses render the chemical bonds more susceptible to chemical attack or corrosion (e.g., stress corrosion). Consequently, microscopic regions are removed from the surface being polished which results in enhancing the planarity of the polished surface.
Presently known polishing techniques are unsatisfactory in several regards. For example, when the slurry is deposited at the polishing pad, the slurry often does not cover the entire surface of the polishing pad, leaving dry spots. In addition, the slurry may not be evenly distributed across the entire surface of the polishing pad leaving a distribution of slurry that is excessive or inadequate in different areas. This uneven distribution negatively effects the material removal rate and wafer uniformity and planarity, thereby resulting in finished workpieces having poor quality. Various modifications to the pad surface to aid in slurry distribution are known in the prior art. These modifications include spiral grooves, v-shaped and square-bottomed grooves, and crosshatched patterns. Although these modifications may assist in the transfer of slurry across the polishing surface, they cause or fail to eliminate other difficulties.
One of these unresolved difficulties is the movement of debris into and out of the grooves in the pad and across the polishing surface. As polishing continues, debris is created either from the removed wafer film materials or from the pad itself (due to conditioning or direct wear during polishing). The debris acts to clog presently known grooved, channeled, and perforated structures in the pad. The slurry is typically in a hydrated state and, if allowed to stagnate in any grooves, channels, or perforations of the polishing surfaces the slurry will begin to dry, aggregate, and pack these structures. If any of this hardened slurry is allowed to reach the workpiece surfaces during polishing, severe scratching of the workpiece can result. This problem is particularly significant in square shaped grooves and perforated structures that are shown in cross-section in FIG. 1. As can be seen in FIG. 1 it is easy for the slurry and debris 130 to enter the hollow features 110 in the polishing surface but difficult for the slurry and debris to exit. This transport limitation results in excessive stagnation of the entrapped material 130 which in turn, leads to aggregation and ultimately to scratching of the workpiece surface. V-shaped channels or grooves, shown in cross-section in FIG. 2, seek to alleviate this and other problems, but do so inadequately and cause further difficulties due to their ability to bi-directionally 230 "pump" the used slurry and debris.
In general, symmetric grooves allow either irregular pumping, no pumping, or bi-directional pumping and all of these effects are undesired. Centripetal acceleration generated by the rotation of the platen and motion of the wafer over the pad provides for the "pumping" of the slurry and debris across the polishing surface. In a square shaped symmetric groove, the vertical walls of the grooves do not allow for transport of the slurry and debris across the polishing surface. Specifically, it is difficult for the slurry and debris to "climb" the vertical wall under the centripetal force provided by the rotation. Furthermore, the motion of the wafer across the grooves in the polishing surface effectively seals the grooves allowing no pumping action or, in the best case scenario, irregular pumping. Therefor, slurry and debris are not evenly transported across the polishing surface. V-shaped grooves with sloped sides allow improved transport under the effects of the centripetal force, however; they permit the motion of the wafer to pump slurry bi-directionally. Furthermore, this bi-directional pumping of the slurry results in the new slurry becoming contaminated with the old slurry and debris.
Any sharp exposed edges or points (asperities) 120, FIG. 1, in the polishing surface that contact the wafer will result in dishing or other process irregularities. These features are omnipresent in square-grooved polishing surfaces. Land areas 140, FIG. 1, or 240, FIG. 2, are the upper surfaces of a grooved, channeled, or perforated surface that contact the workpiece. Land areas that change in shape or area density during the lifetime of the pad or processing cycle of the workpiece also should be avoided. These changes in the land density alter the distribution of slurry and polishing pressure across the surface of the workpiece while the workpiece is in contact with the pad and lead to non-uniform and non-planar polishing and disruption of the long term stability of the batch processing of workpieces. As a V-grooved pad wears, the width of the "V" varies and this trend promotes, rather than corrects, the problems associated with changing land area.
An additional cause of poor polishing performance is the uneven or irregular wear of the polishing surface itself. The polishing process removes material simultaneously from both the workpiece and the polishing pad. If the removal of material from the pad surface is irregular, tracking and formation of a non-planar polishing surface occurs. Tracking is the formation of a nonplanar profile of the polishing surface that is lower (more worn) in the center of the wafer polishing region and higher (less worn) at the periphery of the wafer polishing region. Consequently, the polishing effects that are brought about by aspects of the polishing surface can be non-uniform across the surface of the workpiece, resulting in a non-uniform workpiece surface. Typical prior art polishing pad surface structure designs (e.g., U.S. Pat. No. 5,177,908, assigned to Micron Technology, Inc.) attempt to provide constant surface contact rates between the workpiece and the polishing surface but still allow tracking to occur. Tracking may be partially corrected by repeated excessive conditioning of the pad with abrasives, however; this process significantly reduces the lifetime of the polishing pad thereby increasing usage costs.
For the above detailed reasons and others, apparatus and methods are thus needed which will permit a higher degree of planarization and uniformity over the entire surface of the workpiece. These parameters can be affected by methods and apparatus that uniformly and evenly distribute the slurry across a polishing surface and which provide self-cleaning and anti-tracking of the polishing surface and workpiece.